1. Field of the Invention
The present invention relates to a flux-cored wire containing flux within a stainless steel or mild steel outer cover for use in gas-shielded arc welding and stainless steel welding and a gas-shielded arc welding method using the flux-cored wire.
2. Description of the Related Art
Intermediate Cr ferrite steels containing approximately 14% to 19% by mass Cr are widely used as ferritic stainless steels. In particular, SUS430 steel has various uses, such as for internal building materials and automotive decorative materials, and is manufactured in large quantities.
Because of their high corrosion resistance and heat resistance, ferritic stainless steels are also used in automotive exhaust components for high-temperature gas from engines, such as exhaust manifolds, front pipes, and main mufflers.
Ferritic stainless steels can be welded by a gas-shielded arc welding method. The gas-shielded arc welding method employs roughly two types of welding wires: solid wires and flux-cored wires.
Use of solid wires in gas-shielded arc welding can relatively easily reduce the generation of oxides and can realize excellent high-temperature oxidation resistance. However, solid wires generally contain a small number of non-metallic inclusions, which serve as sites for the formation of crystal grains. Welding using solid wires therefore often forms large crystal grains and causes corrosion in grain boundaries. Thus, gas-shielded arc welding using solid wires has low intergranular corrosion resistance.
In view of such situations, a gas-shielded arc welding method using a flux-cored wire has been proposed for welding of ferritic stainless steels.
For example, Japanese Unexamined Patent Application Publications No. 2001-293596 and No. 2007-319910 propose a flux-cored wire having a specified chemical composition of the wire and use of an Ar—CO2 gas containing 20% CO2 gas as a shielding gas.
Japanese Unexamined Patent Application Publications No. 2007-289965, No. 2007-296535, and No. 2009-255125 propose a flux-cored wire (or a welding method) having a specified chemical composition of the wire and use of pure Ar gas as a shielding gas.
In accordance with the techniques described in Japanese Unexamined Patent Application Publications No. 2001-293596, No. 2007-319910, No. 2007-289965, No. 2007-296535, and No. 2009-255125, use of flux-cored wires can reduce the crystal grain size as compared with solid wires and consequently reduce the occurrence of corrosion at grain boundaries, thereby realizing a certain degree of intergranular corrosion resistance.
However, in accordance with the techniques described in Japanese Unexamined Patent Application Publications No. 2001-293596 and No. 2007-319910, 20% CO2 gas in the Ar—CO2 gas used as a shielding gas limits the improvement in corrosion resistance (in particular, high-temperature oxidation resistance), burn through resistance, and welding performance.
In accordance with the techniques using pure Ar gas as a shielding gas described in Japanese Unexamined Patent Application Publications No. 2007-289965, No. 2007-296535, and No. 2009-255125, a wire contains C and O in order to stabilize an arc. The presence of C and O in the wire limits the improvement in corrosion resistance (in particular, high-temperature oxidation resistance).
CO2 in the shielding gas in the technique described in Japanese Unexamined Patent Application Publications No. 2001-293596 and No. 2007-319910 and C and O in the wire in the technique described in Japanese Unexamined Patent Application Publications No. 2007-289965, No. 2007-296535, and No. 2009-255125 make it difficult to decrease the oxygen content of weld metal, thus possibly causing corrosion in the weld metal at high temperature. In other words, there is room for improvement in corrosion resistance (in particular, high-temperature oxidation resistance) in the techniques described in Japanese Unexamined Patent Application Publications No. 2001-293596, No. 2007-319910, No. 2007-289965, No. 2007-296535, and No. 2009-255125.